Add comment about lack of instance-support in .hi-boot
<listitem>
<para>Only <literal>data</literal>, <literal>type</literal>,
<literal>newtype</literal>, <literal>class</literal>, and
<listitem>
<para>Only <literal>data</literal>, <literal>type</literal>,
<literal>newtype</literal>, <literal>class</literal>, and
- type signature declarations may be included.</para>
+ type signature declarations may be included. You cannot declare
+ <literal>instances</literal> or derive them automatically.
+</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>